The present invention relates to a process for reducing coke formation in organic compound conversion processes.
Fouling of catalysts and/or process equipment by coke is a major problem in high temperature organic compound conversion processes. Coke can cover active catalyst sites and plug catalyst pores, thereby reducing activity. In process equipment, it can build up on furnace tubes and reactors leading to heat transfer and pressure drop problems. Coking in some cases can be so severe as to completely plug the process with coke. While there are many methods of controlling coking such as careful selection of catalysts and plant metallurgy, application of low coking coatings, and/or the addition of steam or sulfur, coking still remains a major problem. Application of low coking coatings, often referred to as Metal Protection Technology (MPT) is taught in U.S. Pat. Nos. 5,866,743; 5,849,969; 6,019,943; 5,676,821; 5,593,571; 5,863,418; and 5,413,700 all of which are incorporated herein by reference. In some processes such as delayed coking or flexi-coking, coke is a by-product of the process that has a very low value. A particular type of coking that is a problem in hydrocarbon processing is Metal Catalyzed Coking. Metal catalyzed coking occurs when hydrocarbons and/or carbon monoxide present in certain processes react with the plant or process metallurgy at temperatures typically above 800xc2x0 F. to produce carbon-containing deposits. These carbon containing deposits can build up to a level where they are detrimental to the process by creating, for example, pressure drop problems, blocking off catalytic sites, and/or impeding the transfer of heat, in for example, a furnace tube. Metal catalyzed coking is also an indication that the plant or process metallurgy is undergoing metal dusting and possibly carburization. Both of these later processes can affect the structural integrity of the metallurgy. Typically, iron, cobalt and/or nickel containing alloys show metal catalyzed coking at temperatures of 800xc2x0 F. or higher. Of particular interest to this invention are high temperature processes such as steam reforming, partial oxidative reforming, and/or autothermal reforming associated with the conversion of hydrocarbons to carbon monoxide and hydrogen for use in Fischer-Tropisch plants, syngas to methanol plants, fuel cells or any other process that require or consume hydrogen and/or carbon monoxide. There are many methods taught in the art to control metal catalyzed coking. Some techniques such as addition of sulfur to the process stream cannot be used in certain processes such as steam or autothermal reforming processes due to sulfur poisoning of the reformer catalyst. Thus the addition of steam is normally used to control coking. Typically, the steam to carbon mole ratio used in reforming processes range from about 0.5 to 6 and more commonly from about 2 to 4. However addition of steam is frequently not sufficient to control metal catalyzed coking. In one form of the art, iron, cobalt and/or nickel containing alloys are treated with aluminum at high temperature to form aluminum diffusion coating. Methods for preparing such coatings are taught in the xe2x80x9cMetals Handbookxe2x80x9d, 9th Ed, Vol 5 page 611-613 and for example in U.S. Pat. Nos. 1,988,217, 3,486,927 and 3,254,969 all of which are incorporated herein by reference. U.S. Pat. No. 1,988,217, to Sayles, which is incorporated herein by reference, teaches that aluminum diffusion coatings with a surface concentration of about 5 to 35% Al can be used to protect oil furnace tubes and other high temperature processes from the corrosive action of oil. U.S. Pat. No. 3,827,967 to Nap et. al. which is incorporated herein by reference, teaches that an aluminum coated alloy will resist coking in the thermal cracking of hydrocarbon feedstocks. The beneficial effects of aluminum diffusion coatings on specifically suppressing metal catalyzed coking under low sulfur conditions is claimed in U.S. Pat. No. 5,849,969 to Heyse et. al. Alternatively, aluminum-containing alloys have been shown to be resistant to coking. U.S. Pat. No. 4,532,109 to Maeda teaches that alloys containing 1-10% Al that have been oxidized prior to high temperature contact with hydrocarbons, or carbon monoxide show reduced metal catalyzed coking rates. U.S. Pat. Nos. 4,532,109, and 5,849,969 are incorporated herein by reference in their entirety. Unfortunately, metal catalyzed coking can still occur on Al containing alloys or Al rich surfaces under conditions commonly encountered in high temperature processes. Clearly any method that can reduce the amount of metal catalyzed coke formed would be very beneficial. The present invention provides such a method that can be used to minimize coking in a wide variety of processes and applications.
The present invention provides a process for reducing metal catalyzed coke formation in organic compound conversion processes. The present invention dramatically reduces or eliminates metal catalyzed coking on aluminum coated or aluminum containing nickel and/or cobalt containing alloys by the presence of carbon dioxide and steam in the process stream.
One embodiment of the present invention describes a process for steam reforming of hydrocarbons to produce hydrogen and carbon monoxide, comprising: passing a feed comprising hydrocarbons, steam, and CO2 over a steam reforming catalyst, at steam reforming conditions comprising a temperature of at least 800 degrees F., in a steam reformer to form an effluent comprising CO and hydrogen, wherein said steam reformer is constructed at least in part of a material comprising of a cobalt or nickel containing alloy further comprising aluminum or an aluminum coating, cladding, or paint.
Another embodiment of the present invention involves a method for prevention of metal catalyzed coking in a reforming process producing carbon monoxide and hydrogen, comprising constructing the reformer using alloys comprising a metal selected from the group consisting of nickel and cobalt, said alloy also comprising aluminum or being coated, cladded, plated or painted with a material comprising aluminum, and; feeding a hydrocarbon to said reforming process comprising at least 0.5% CO2 by volume and a steam to carbon mole ratio of at least 0.3.
The present invention also describes a process for forming hydrogen for use in a fuel cell comprising: passing a feed comprising hydrocarbons, and CO2, in combination with steam in a reforming process in the presence of a nickel or cobalt containing alloy, at reforming conditions sufficient to form carbon monoxide and hydrogen, wherein said alloy further comprises aluminum or an aluminum diffusion layer and said CO2 content in the feed is sufficient to suppress metal catalyzed coking.
A particularly preferred embodiment of the present invention describes a process for forming hydrogen for use in a fuel cell, comprising: passing a feed comprising hydrocarbons, and CO2, in combination with steam in a reforming process in the presence of a nickel and/or cobalt containing alloy, at reforming conditions sufficient to form carbon monoxide and hydrogen, wherein said alloy further comprises aluminum or an aluminum diffusion layer and said CO2 content in the feed is sufficient to suppress metal catalyzed coking.
In an alternative embodiment of the present invention describes a process for forming hydrogen and carbon monoxide for use in a Fischer-Tropsch plant, comprising: passing a feed comprising hydrocarbons, and CO2, in combination with steam in a reforming process in the presence of a nickel or cobalt containing alloy, at reforming conditions sufficient to form carbon monoxide and hydrogen, wherein said alloy further comprises aluminum or an aluminum diffusion layer and said CO2 content in the feed is sufficient to suppress metal catalyzed coking.
Among other factors I have surprisingly discovered that metal catalyzed coking can be suppressed if not completely eliminated by the use of a nickel and/or cobalt containing alloy that has been aluminum coated or contains aluminum when a controlled amount of carbon dioxide and steam are present in the process stream.
I have discovered that metal catalyzed coking is dramatically reduced or eliminated on aluminum coated or aluminum containing nickel and/or cobalt containing alloys if carbon dioxide and steam is present in the process stream. This is actually quite surprising since carbon dioxide is known to react with hydrocarbons over nickel containing catalysts to form carbon monoxide and hydrogen in a process known as dry reforming. Coking is a serious problem in dry reforming. Thus, I was quite surprised to discover that low levels of carbon dioxide greater than about 0.5 vol % in the process stream, additionally containing steam, will actually suppress coking on Ni and/or cobalt containing alloys containing at least 8 wt % Ni and/or Co and additionally containing aluminum either present in the alloy in the range from about 1 to 10 wt % or as an aluminum surface diffusion coating. The level of carbon dioxide that is needed to prevent coking is directly dependent upon the level of Ni and or Co in the alloy and indirectly proportional to the concentration of Al in the alloy. Thus a high nickel and/or cobalt containing alloy will need a higher concentration of carbon dioxide in the process stream to prevent metal catalyzed coking if the concentration of aluminum in the alloy or the surface diffusion layer is low. Alternatively, a high concentration of aluminum on the outer surface of the alloy will require less carbon dioxide in the process stream to prevent or reduce metal catalyzed coking. Additionally, steam also needs to be present in the process with a steam to carbon ratio greater than about 0.3 in order for carbon dioxide and aluminum in the alloy or as a surface diffusion coating to be effective in suppressing metal catalyzed coking.
Furthermore, I have discovered that it is important that if low Ni and cobalt alloys are used in this art that the concentration of carbon dioxide in the process stream must be in the high range in order to prevent metal catalyzed coking. I have also found that high nickel or cobalt alloys coated with or containing aluminum exhibit better long term durability in reforming applications containing carbon dioxide than does low nickel alloys. Thus high nickel and cobalt alloys are preferred in this art. The sum of nickel plus cobalt should preferably be greater than 8 wt %, more preferably greater than 10 wt %, more preferably greater than 14%, and even more preferably greater than 40%.